This is an open-label, multiple-dose study to evaluate the safety and efficacy of BG9588 in up to 23 subjects with chronic refractory immune thrombocytopenia (ITP) who previously participated in Biogen study C96-1000. Subjects will receive BG9588 via a 15 min IV infusion every 4 weeks for a total of up to 13 doses. The initial dose will be 3mg/kg continued for three consecutive treatment cycles. Subsequent dosing will be based on the platelet response during the third of three consecutive treatment cycles as follows: 1) those whose counts were greater than 50K on both the Day 1 and Day 15 determinations of the third treatment cycle will have their dose reduced; 2) those with both the Day 1 and Day 15 platelet count less than 30K will have their dose increased; 3) all others will maintain their dose. The maximum dose will not exceed 10 mg/kg. The BG9588 dosing may be delayed for subjects with major bleeding events or other medical events such as infection.